1. Field of the Invention
This invention relates to an engine provided with a mechanical supercharger which is driven by engine output power.
2. Description of the Prior Art
In an engine provided with a supercharger, the supercharger is generally driven during the operation of the engine under a heavy load in order to increase the engine output power. When the engine is supercharged, the pressure and the temperature in the combustion chamber increase, and the engine enters a state in which it may knock Knocking of the engine during supercharging can be suppressed by extending the valve overlap time, that is, the time both the intake valve and the exhaust valve are open, thereby lowering the temperature in the combustion chamber and the effective compression rate, as disclosed in Japanese Unexamined Patent Publication Nos. 61(1986)-197720 and 61(1986)-187543 and U.S. patent application Ser. No. 125,982. On the other hand, when the valve overlap time is long during the operation of the engine under a light load when the supercharger is not operated, e.g., idling, exhaust gas blows back into the combustion chamber under negative intake pressure and an excessive amount of exhaust gas mingles with the incoming air-fuel mixture, thereby deteriorating combustion in the combustion chamber. Accordingly, in accordance with the prior art, the valve overlap time is extended only when the supercharger is operated so that the amount of exhaust gas mingling with the incoming air-fuel mixture is reduced during the operation of the engine under a light load. However, it is known in the art that when the valve overlap time is long, a part of exhaust gas mingles with the incoming air-fuel mixture and an effect similar to exhaust gas recirculation can be obtained, thereby improving the fuel economy, and the supercharged engine in accordance with the prior art is disadvantageous in that this effect cannot be enjoyed during the operation of the engine under a mid-range load.